Abstract
According to the ancient doctrines of Democritus and Epicurus, atoms are tiny indivisible masses rattling about through the vacuum of space. This atomic hypothesis was preserved—in one form or another—for over 2000 years. Yet by the close of the nineteenth century it was still not universally accepted. The most convincing evidence for atomism was rather indirect, having come out of (i) the study of chemical reactions, (ii) the kinetic theory of gases and (iii) Boltzmann’s probabilistic interpretation of entropy. Even among the atomists there was considerable disagreement regarding the nature and structure of the atom itself. For example, the followers of Roger Boscovich believed atoms to be little more than point-like mathematical centers of force. Boscovich’s atoms possessed an unchangeable mass, they could move through the vacuum of space, and they were endowed with an irreducible power to attract or repel other atoms. A very different view of the atom was maintained byWilliam Thomson (Lord Kelvin). Inspired by the earlier work of Hermann von Helmholtz, Kelvin imagined atoms to be like minuscule smoke-rings traveling through an all-pervasive frictionless fluid medium—the æther. These so-called vortex ring atoms were indivisible local excitations which could emit light by vibrating like the rim of a tiny ringing bell. In this way, the vortex ring model of the atom offered the possibility of explaining the complicated emission spectra of atomic gasses.
Kelvin’s vortex theory of the atom aroused the interest of another Thomson, J.J., who published his Treatise on the motion of vortex rings in 1883 while studying at the University of Cambridge. A few years later, J.J. Thomson would propose the existence of subatomic particles which weighed considerably less than the smallest known atom. In the reading selection below, Thomson describes his famous 1897 discovery of subatomic electrically charged particles. It is taken from the first chapter of his 1907 book on The Corpuscular Theory of Matter.
The atom is not the ultimate limit to the subdivision of matter; we may go further and get to the corpuscle, and at this stage the corpuscle is the same from whatever source it may be derived.
—J. J. Thomson
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Notes
- 1.
- 2.
Roger Boscovich was an eighteenth century natural philosopher and Jesuit priest.
- 3.
See Part I, Sect. 7 of Boscovich, R. J., A Theory of Natural Philosophy, Open Court Publishing Co., Chicago and London, 1922. Boscovich originally published this work in 1763.
- 4.
See Thomson, W., On Vortex Atoms, Proceedings of the Royal Society of Edinburgh, 6, 1867.
- 5.
Niels Bohr discusses atomic emission spectra in the context of his quantized model of the atom; see Chap. 28 of the present volume.
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- 7.
See, for example, Davisson’s paper on electron diffraction which is included in Chap. 26 of the present volume.
- 8.
I have had success with two different pieces of equipment: the Nakamura Model B10-7350, and the Sargent Welch Model P63412. Detailed instructions on the operation of these pieces of equipment are included with the equipment. The most expensive component of the apparatus is the bulb, which costs about 800 USD. In addition, you will need a low-voltage power supply to heat the lamp filament, a high-voltage (500 V) power supply to accelerate the electrons, current source (3 amps) to drive the helmholtz coils, and a hand-held digital multimeter.
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Kuehn, K. (2016). The Discovery of the Electron. In: A Student's Guide Through the Great Physics Texts. Undergraduate Lecture Notes in Physics. Springer, Cham. https://doi.org/10.1007/978-3-319-21828-1_17
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DOI: https://doi.org/10.1007/978-3-319-21828-1_17
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